Separators for electrochemical cells

ABSTRACT

Provided are separators for use in an electrochemical cell comprising (a) an inorganic oxide and (b) an organic polymer, wherein the inorganic oxide comprises organic substituents. Also provided are electrochemical cells comprising such separators.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.17/406,515, filed on Aug. 19, 2021 (now U.S. Pat. No. 11,264,676), whichis a Continuation of U.S. Patent Application Ser. No. 17/019,584, filedon Sep. 14, 2020 (now U.S. Pat. No. 11,121,432), which is a Continuationof U.S. patent application Ser. No. 16/657,257, filed on Oct. 18, 2019(now U.S. Pat. No. 10,797,288), which is a Continuation of U.S. patentapplication Ser. No. 15/799,449, filed Oct. 31, 2017 (now U.S. Pat. No.10,505,168), which is a Continuation of U.S. patent application Ser. No.14/534,991, filed Nov. 6, 2014 (now U.S. Pat. No. 9,871,239), which is aContinuation of U.S. patent application Ser. No. 11/652,948, filed Jan.12, 2007 (now U.S. Pat. No. 8,883,354), which claims the benefit of andpriority to U.S. Provisional Application Ser. No. 60/773,487, filed Feb.15, 2006, the contents of each of which are incorporated by reference asif fully set forth herein.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with government support under Grant NumberDE-FG02-02ER83542 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to the field of porous membranesand to the fields of electrochemical cells and of separators for use inelectrochemical cells. More particularly, this invention pertains to aporous separator membrane comprising an organically-modified inorganicoxide and an organic polymer. Also, the present invention pertains toelectrochemical cells comprising such porous separators.

BACKGROUND

Throughout this application, various publications, patents, andpublished patent applications are referred to by an identifyingcitation. The disclosures of the publications, patents, and publishedpatent specifications referenced in this application are herebyincorporated by reference into the present disclosure to more fullydescribe the state of the art to which this invention pertains.

An electroactive material that has been fabricated into a structure foruse in an electrochemical cell is referred to as an electrode. Of a pairof electrodes used in an electrochemical cell, the electrode on theelectrochemically higher potential side is referred to as the positiveelectrode or the cathode, while the electrode on the electrochemicallylower potential side is referred to as the negative electrode, or theanode. A battery may contain one or more electrochemical cells.

An electrochemically active material used in the cathode or positiveelectrode is referred to hereinafter as a cathode active material. Anelectrochemically active material used in the anode or negativeelectrode is hereinafter referred to as an anode active material. Anelectrochemical cell comprising a cathode with the cathode activematerial in an oxidized state and an anode with the anode activematerial in a reduced state is referred to as being in a charged state.Accordingly, an electrochemical cell comprising a cathode with thecathode active material in a reduced state, and an anode with the anodeactive material in an oxidized state, is referred to as being in adischarged state.

Discharging an electrochemical cell in its charged state by allowingelectrons to flow from the anode to the cathode through an externalcircuit results in the electrochemical reduction of the cathode activematerial at the cathode and the electrochemical oxidation of the anodeactive material at the anode. To prevent the undesirable flow of theelectrons in a short circuit internally from the anode to the cathode,an electrolyte element is interposed between the cathode and the anode.This electrolyte element must be electronically non-conductive toprevent short circuits, but must permit the transport of ions betweenthe anode and the cathode. The electrolyte element should also be stableelectrochemically and chemically toward both the anode and the cathode.

Typically, the electrolyte element contains a porous material, referredto as a separator (since it separates or insulates the anode and thecathode from each other), and an aqueous or non-aqueous electrolyte,that usually comprises an ionic electrolyte salt and ionicallyconductive material, in the pores of the separator. A variety ofmaterials have been used for the porous layer or separator of theelectrolyte element in electrochemical cells. These porous separatormaterials include polyolefins such as polyethylenes and polypropylenes,glass fiber filter papers, and ceramic materials. Usually theseseparator materials are supplied as porous free-standing membranes thatare interleaved with the anodes and the cathodes in the fabrication ofelectrochemical cells. Alternatively, the porous separator may beapplied directly to one of the electrodes, for example, as described inU.S. Pat. No. 5,194,341 to Bagley et al., and in U.S. Pat. No. 6,153,337to Carlson et al.

Porous separator materials have been fabricated by a variety ofprocesses including, for example, stretching combined with specialheating and cooling of plastic films, extraction of a solubleplasticizer or filler from plastic films, and plasma oxidation. Themethods for making existing free-standing separators typically involvethe extrusion of melted polymeric materials either followed by apost-heating and stretching or drawing process or followed by a solventextraction process to provide the porosity throughout the separatorlayer. U.S. Pat. No. 5,326,391 to Anderson et al., and referencestherein, describe the fabrication of free-standing porous materialsbased on extraction of a soluble plasticizer from pigmented plasticfilms. U.S. Pat. No. 5,418,091 to Gozdz et al., and references therein,describe forming electrolyte layers by extracting a soluble plasticizerfrom a fluorinated polymer matrix either as a coated component of amultilayer battery structure or as an individual separator film.

A liquid organic electrolyte containing organic solvents and lithiumsalts is typically used as the electrolyte in the pores of the separatorin the electrolyte element for lithium-ion electrochemical cells.Alternatively, a gel or solid polymer electrolyte containing anionically conductive polymer and lithium salts, and optionally organicsolvents, might be utilized instead of the liquid organic electrolyte.For example, U.S. Pat. Nos. 5,597,659 and 5,691,005 to Morigaki et al.describe a separator matrix formed of a microporous polyolefin membraneimpregnated in its pores with an ionic conductive gel electrolyte.

In addition to being porous and chemically stable to the other materialsof the electrochemical cell, the separator should be flexible, thin,economical in cost, and have good mechanical strength. These propertiesare particularly important when the cell is spirally wound or is foldedto increase the surface area of the electrodes and thereby improve thecapacity and high rate capability of the cell. Typically, free-standingseparators have been 20 microns or greater in thickness. As lithium-ionbatteries have continued to evolve to higher volumetric capacities andsmaller lightweight structures, there is a need for separators that are15 microns or less in thickness. Reducing the thickness from 20 micronsto 15 microns or less greatly increases the challenge of providing highporosity and good mechanical properties while not sacrificing theprotection against short circuits or not significantly increasing thetotal cost of the separator in each battery.

High porosity in the separator is important for obtaining the high ionicconductivity needed for effective performance in most batteries, except,for example, those batteries operating at relatively low charge anddischarge rates. It is desirable for the separator to have a porosity ofat least 45 percent, and preferably 50 percent or higher, in lithium-ionbatteries. As the separator is reduced in thickness from the typical 20to 25 microns to 15 microns or less, the approximately 50 percent solidsvolume of the separator that is not voids or pores, must contribute allof the mechanical properties needed for fabrication into theelectrochemical cell and for mechanical integrity during the storage andoperation of the battery. Typically, lowering the porosity to increasethe mechanical properties also reduces the ionic conductivity. Thistrade-off between high conductivity and good mechanical properties is achallenge in providing separators that are less than 25 microns inthickness, especially for those that are less than 15 microns thick.

The protection against short circuits is particularly critical in thecase of secondary or rechargeable batteries with lithium as the anodeactive material. During the charging process of the battery, dendritesmay form on the surface of the lithium anode and may grow with continuedcharging. A key feature of the separator in the electrolyte element oflithium-ion rechargeable batteries is that it has a small porestructure, such as 0.5 microns or less in pore diameter, and sufficientmechanical strength to prevent the lithium dendrites from contacting thecathode and causing a short circuit with perhaps a large increase in thetemperature of the battery leading to an unsafe condition.

Another highly desirable feature of the separator in the electrolyteelement is that it is readily wetted by the electrolyte materials thatprovide the ionic conductivity. When the separator material is apolyolefin material that has non-polar surface properties, theelectrolyte materials (which typically have highly polar properties)often poorly wet the separator material. This results in longer times tofill the battery with electrolyte and potentially in low capacities inthe battery due to a non-uniform distribution of electrolyte materialsin the electrolyte element.

Further, it would be highly advantageous to be able to prepareseparators by a relatively simple process of coating that directlyprovides ultrafine pores less than 50 nm in diameter and can readilyprovide a range of thicknesses from 40 microns or greater down to 1micron.

A separator, particularly one with a thickness less than 15 microns,that is applicable for lithium-ion and other electrochemical cells, andthat can reduce the trade-off between high ionic conductivity and goodmechanical properties, would be of great value to the battery industry.

SUMMARY OF THE INVENTION

To achieve high porosity and high ionic conductivity while providinggood strength and flexibility in separators for use in electrochemicalcells, the present invention utilizes organically-modified inorganicoxides in the separators and utilizes various mixing, coating, drying,delaminating, and laminating methods for preparing such separators.

One method of the present invention for preparing a separator for anelectrochemical cell comprises the steps of (a) coating onto a substratea liquid mixture comprising an inorganic oxide, an organic polymer, andan organic compound, preferably a multifunctional monomer or an organiccarbonate; (b) drying the coating formed in step (a) to yield amicroporous inorganic oxide layer, preferably a xerogel layer; and (c)delaminating the inorganic oxide layer from the substrate to form theseparator, wherein the separator comprises the microporous inorganicoxide layer having pores connected in a substantially continuous fashionthrough the layer. In a preferred embodiment, the inorganic oxide instep (b) and step (c) comprises organic substituents. In one embodiment,the organic substituents comprise a reaction product of the organiccompound, preferably a multifunctional monomer and/or an organiccarbonate, with the inorganic oxide of step (a). In one embodiment, theinorganic oxide of step (c) comprises a hydrated aluminum oxide of theformula Al₂O₃.xH₂O, wherein x is less than 1.0, and wherein the hydratedaluminum oxide comprises organic substituents, preferably comprising areaction product of a multifunctional monomer and/or organic carbonatewith the inorganic oxide of step (a), such as, for example,pseudo-boehmite. In one embodiment, x is less than 0.8. In oneembodiment, x is less than 0.6.

In one embodiment of the methods of preparing a separator of the presentinvention, the separator formed in step (c) is a free-standing porousmembrane comprising the inorganic oxide layer, preferably a xerogellayer. In another embodiment, the inorganic oxide layer of step (b) islaminated to an electrode for an electrochemical cell prior to step (c)and the delamination of step (c) forms the separator laminated to theelectrode.

In one embodiment of the methods, separators, and cells of thisinvention, the porosity of the separator is from 48 percent to 62percent. In one embodiment, the elastic modulus of the separator is from15,000 kg/cm² to 50,000 kg/cm². In one embodiment, the elastic modulusof the separator is from 30,000 kg/cm² to 70,000 kg/cm². In anotherembodiment, the tensile strength of the separator at 2 percentelongation is from 100 kg/cm² to 500 kg/cm². In one embodiment, thepercent elongation of the separator at break is from 2 percent to 10percent. In one embodiment, the percent elongation of the separator atbreak is from 5 percent to 20 percent. In one embodiment, the percentelongation of the separator at break is greater than 10 percent. In oneembodiment, the percent elongation of the separator at break is greaterthan 15 percent. In one embodiment, the separator does not melt attemperatures lower than 300° C.

In one embodiment of the methods, separators, and cells of thisinvention, the pore volume of the separator is from 48 percent to 62percent, the elastic modulus of the separator is greater than 30,000kg/cm², and the percent elongation of the separator at break is greaterthan 5 percent. In one embodiment, the pore volume of the separator isfrom 48 percent to 62 percent, the elastic modulus of the separator isgreater than 30,000 kg/cm², and the percent elongation of the separatorat break is greater than 10 percent.

In one embodiment of the methods, separators, and cells of thisinvention, the average pore diameter of the inorganic oxide layer,preferably a xerogel layer, is from 2 nm to 70 nm. In one embodiment,the organic polymer is present in the amount of 5 percent to 35 percentof the weight of the organically-modified inorganic oxide in theinorganic oxide layer. In one embodiment, the organic polymer comprisesa polymer selected from the group consisting of polyvinyl alcohols,polyethylene oxides, polyvinyl pyrrolidones, and cellulosic polymers. Inone embodiment of the methods of the present invention, the inorganicoxide of step (a) is selected from the group consisting ofpseudo-boehmites, aluminum oxides, silicon oxides, tin oxides, titaniumoxides, and zirconium oxides.

In one embodiment of the methods of preparing a separator of the presentinvention, the drying of step (b) comprises drying at a temperaturegreater than 150° C. In one embodiment, the drying at a temperaturegreater than 150° C. increases the tensile strength of the separator at2 percent elongation and increases the percent elongation of theseparator at break compared to drying for the same period of time at atemperature of 140° C. or less. In one embodiment, the methods furthercomprise a step (d) of drying at a temperature greater than 150° C.

In one embodiment of the methods, separators, and cells of thisinvention, the substrate is a silicone release substrate. In oneembodiment, the liquid mixture further comprises a surfactant. In oneembodiment, the surfactant comprises a fluorosurfactant. In oneembodiment, the inorganic oxide layer, preferably a xerogel layer, has athickness from 2 microns to 25 microns.

Other aspects of this invention are separators prepared by the methodsof this invention. In one embodiment of the separators of thisinvention, the separator comprises a microporous layer comprising (a) aninorganic oxide and (b) an organic polymer, wherein the inorganic oxidecomprises organic substituents. In one embodiment, the organicsubstituents comprise a reaction product of an organic compound,preferably a multifunctional monomer and/or an organic carbonate, withthe inorganic oxide. In one embodiment, the inorganic oxide is ahydrated aluminum oxide of the formula Al₂O₃.xH₂O, wherein x is lessthan 1.0, and wherein the hydrated aluminum oxide comprises organicsubstituents, preferably comprising a reaction product of amultifunctional monomer and/or organic carbonate with an inorganicoxide, such as, for example, pseudo-boehmite. In one embodiment, x isless than 0.8. In one embodiment, x is less than 0.6.

Still other aspects of the present invention are electrochemical cellscomprising the separators prepared by the methods of this invention. Inone embodiment of the electrochemical cells of the present invention,the electrochemical cell comprises an anode, a cathode, and a separatorof the present invention interposed between the anode and the cathode,wherein the separator comprises a microporous layer comprising (a) aninorganic oxide and (b) an organic polymer, wherein the inorganic oxidecomprises organic substituents. In one embodiment, the cell compriseslithium as the anode active material. In one embodiment, the cell is asecondary or rechargeable cell. In one embodiment, the cell is a primaryor non-rechargeable cell.

DETAILED DESCRIPTION OF THE INVENTION

The separators and methods of preparing separators of the presentinvention provide superior properties of ionic conductivity, porosity,strength, and flexibility for use in electrochemical cells, particularlyin cells utilizing separators with thicknesses below about 15 microns.

One method of the present invention for preparing a separator for anelectrochemical cell comprises the steps of (a) coating onto a substratea liquid mixture comprising an inorganic oxide, an organic polymer, anda divinyl ether of an ethylene glycol; (b) drying the coating formed instep (a) to yield a microporous inorganic oxide layer, preferably axerogel layer; and (c) delaminating the inorganic oxide layer from thesubstrate to form the separator. In one embodiment, the separatorcomprises a microporous inorganic oxide layer having pores connected ina substantially continuous fashion through the layer. In a preferredembodiment, the inorganic oxide in step (b) and step (c) comprisesorganic substituents. In one embodiment, the organic substituentscomprise a reaction product of the divinyl ether with the inorganicoxide of step (a). As used herein, the term “reaction product” means aproduct from a reaction that formed covalent bonds, ionic bonds,hydrogen bonds, or surface adsorption between two materials. In oneembodiment, the inorganic oxide of step (c) comprises a hydratedaluminum oxide of the formula Al₂O₃.xH₂O, wherein x is less than 1.0,and wherein the hydrated aluminum oxide comprises organic substituents,preferably comprising a reaction product of the divinyl ether with theinorganic oxide of step (a), such as, for example, pseudo-boehmite. Inone embodiment, x is less than 0.8. In one embodiment, x is less than0.6.

Another method of the present invention for preparing a separator for anelectrochemical cell comprises the steps of (a) coating onto a substratea liquid mixture comprising an inorganic oxide, an organic polymer, andan organic carbonate; (b) drying the coating formed in step (a) to yielda microporous inorganic oxide layer, preferably a xerogel layer; and (c)delaminating the inorganic oxide layer from the substrate to form theseparator. In one embodiment, the separator comprises the microporousinorganic oxide layer having pores connected in a substantiallycontinuous fashion through the layer. In a preferred embodiment, theinorganic oxide in step (b) and step (c) comprises organic substituents.In one embodiment, the organic substituents comprise a reaction productof the organic carbonate with the inorganic oxide of step (a). In oneembodiment, the inorganic oxide of step (c) comprises a hydratedaluminum oxide of the formula Al₂O₃.xH₂O, wherein x is less than 1.0,and wherein the hydrated aluminum oxide comprises organic substituents,preferably comprising a reaction product of the organic carbonate withthe inorganic oxide of step (a), such as, for example, pseudo-boehmite.In one embodiment, x is less than 0.8. In one embodiment, x is less than0.6.

Still another method of the present invention for preparing a separatorfor an electrochemical cell comprises the steps of (a) coating onto asubstrate a liquid mixture comprising an inorganic oxide, an organicpolymer, a divinyl ether of an ethylene glycol, and an organiccarbonate; (b) drying the coating formed in step (a) to yield amicroporous inorganic oxide layer, preferably a xerogel layer; and (c)delaminating the inorganic oxide layer from the substrate to form theseparator. In one embodiment, the separator comprises the microporousinorganic oxide layer having pores connected in a substantiallycontinuous fashion through the layer. In a preferred embodiment, theinorganic oxide in step (b) and step (c) comprises organic substituents.In one embodiment, the organic substituents comprise a reaction productof the divinyl ether and/or the organic carbonate with the inorganicoxide of step (a). In one embodiment, the inorganic oxide of step (c)comprises a hydrated aluminum oxide of the formula Al₂O₃.xH₂O, wherein xis less than 1.0, and wherein the hydrated aluminum oxide comprisesorganic substituents, preferably comprising a reaction product of thedivinyl ether and/or the organic carbonate with the inorganic oxide ofstep (a), such as, for example, pseudo-boehmite. In one embodiment, x isless than 0.8. In one embodiment, x is less than 0.6.

One method of the present invention for preparing a separator for anelectrochemical cell comprises the steps of (a) coating onto a substratea liquid mixture comprising an inorganic oxide, an organic polymer, anda divinyl ether of an ethylene glycol; (b) drying the coating formed instep (a) to yield a microporous inorganic oxide xerogel layer; and (c)delaminating the inorganic oxide xerogel layer from the substrate toform the separator, wherein the separator comprises the microporousinorganic oxide xerogel layer having pores connected in a substantiallycontinuous fashion through the xerogel layer. Another method of thepresent invention for preparing a separator for an electrochemical cellcomprises the steps of (a) coating onto a substrate a liquid mixturecomprising an inorganic oxide, an organic polymer, and an organiccarbonate; (b) drying the coating formed in step (a) to yield amicroporous inorganic oxide xerogel layer; and (c) delaminating theinorganic oxide xerogel layer from the substrate to form the separator,wherein the separator comprises the microporous inorganic oxide xerogellayer having pores connected in a substantially continuous fashionthrough the xerogel layer.

Still another method of the present invention for preparing a separatorfor an electrochemical cell comprises the steps of (a) coating onto asubstrate a liquid mixture comprising an inorganic oxide, an organicpolymer, a divinyl ether of an ethylene glycol, and an organiccarbonate; (b) drying the coating formed in step (a) to yield amicroporous inorganic oxide xerogel layer; and (c) delaminating theinorganic oxide xerogel layer from the substrate to form the separator,wherein the separator comprises the microporous inorganic oxide xerogellayer having pores connected in a substantially continuous fashionthrough the xerogel layer. Typically, the liquid mixture will alsocomprise water.

Methods of preparing microporous xerogel separators for electrochemicalcells are described in U.S. Pat. Nos. 6,153,337 and 6,306,545, and inU.S. Patent Application 20020092155, all to Carlson et al. The liquidmixture described in these references for coating xerogel separatorscomprises an inorganic oxide, an organic binder, and typically water asthe volatile liquid in the mixture. Optionally, the liquid mixturecomprises organic solvents, preferably protic organic solvents. Examplesof protic organic solvents are alcohols and glycols.

In the instant invention, it has been found that the presence of adivinyl ether of an ethylene glycol or the presence of an organiccarbonate, or the presence of a combination of both a divinyl ether ofan ethylene glycol and an organic carbonate, in the liquid mixturecomprising an inorganic oxide and an organic polymer producesmicroporous separators, including microporous xerogel separators, havingsignificantly improved ionic conductivity and improved strength andflexibility properties, compared to separators prepared without one ofthese divinyl ether or organic carbonate materials in the liquidmixture. In a preferred embodiment, the divinyl ether and/or organiccarbonate material reacts with the inorganic oxide, such as boehmite, toform a new material, an organically-modified inorganic oxide.

The drying process to form a xerogel layer involves the removal of theliquid in the liquid mixture. As is known in the art of inorganic oxidexerogel coatings, as the liquid is removed, the colloidal particles ofinorganic oxide sol form a gel that, upon further loss of liquid, formsa 3-dimensional microporous network of inorganic oxide. By the terms“xerogel layer” and “xerogel structure,” as used herein, is meant,respectively, a layer of a coating or the structure of a coating layerin which the layer and structure were formed by drying a liquid sol orsol-gel mixture to form a solid gel matrix as, for example, described inChem. Mater., Vol. 9, pages 1296 to 1298 (1997) by Ichinose et al. forcoating layers of inorganic oxide based xerogels. Thus, if the liquid ofthe gel formed in the liquid sol-gel mixture is removed substantially,for example, through the formation of a liquid-vapor boundary phase, theresulting gel layer or film is termed, as used herein, a xerogel layer.Thus, the microporous xerogel layers of this invention comprise a driedmicroporous three-dimensional solid network with pores which areinterconnected in a substantially continuous fashion from one outermostsurface of the layer through to the other outermost surface of thelayer. A continuous xerogel coating layer has the materials of thexerogel in a continuous structure in the coating layer, i.e., thematerials, such as the organically-modified inorganic oxide particles,are in contact and do not have discontinuities in the structure, such asa discontinuous layer of solid pigment particles that are separated fromeach other. In contrast, xerogel pigment particles may be formed by axerogel process involving drying a liquid solution of a suitableprecursor to the pigment to form a dried mass of xerogel pigmentparticles, which is typically then ground to a fine powder to provideporous xerogel pigment particles. The microporous organically-modifiedinorganic oxide layers of this invention may be, but are not limited to,xerogel layers. The organically-modified inorganic oxide layers of thepresent invention may also be discontinuous layers of solid pigmentparticles that are not a xerogel coating layer and have discontinuitiesof solid pigment particles that are separated from each other in thestructure of the discontinuous layer. This separation typically involvesorganic polymer interposed between the pigment particles. The terms“xerogel coating” and “xerogel coating layer,” as used herein, aresynonymous with the term “xerogel layer.”

As used herein, the term “microporous” describes the material of a layeror coating, in which the material possesses pores of a diameter of about1 micron or less. As used herein, the term “nanoporous” describes thematerial of a layer or coating, in which the material possesses pores ofa diameter of about 100 nanometers or less.

Preferably for battery separator applications, these pores are connectedin a substantially continuous fashion from one outermost surface of thexerogel layer through to the other outermost surface of the layer. Thissubstantially continuous 3-dimensional microporous inorganic oxidenetwork is efficient in allowing the diffusion of ions, such as lithiumions, through the separator during the charging and discharging of theelectrochemical cell.

The delamination in step (c) is not limited to xerogel or toorganically-modified inorganic oxide microporous layers that aredirectly coated onto the substrate and includes any separator thatcomprises a xerogel layer or comprises an organically-modified inorganicoxide microporous layer. As noted above, the organically-modifiedinorganic oxide microporous layers of this invention may be eitherxerogel layers or non-xerogel layers with a discontinuous layer of solidpigment particles that are separated from each other. Thus, there may beone or more other types of layers, preferably microporous layers,between the xerogel or the organically-modified inorganic oxide layerand the substrate. Similarly, there may be one or more other types oflayers, preferably microporous layers, on the side of the xerogel or theorganically-modified inorganic oxide layer opposite from the substrate.

In one embodiment of the separators and of the methods of preparing aseparator of the present invention, the separator formed in step (c) isa free-standing porous membrane comprising the inorganic oxide xerogellayer. In one embodiment, the separator formed in step (c) is afree-standing porous membrane comprising an inorganic oxide and anorganic polymer, wherein the inorganic oxide comprises organicsubstituents. Typically, in order to have sufficient mechanical strengthfor fabrication into an electrochemical cell by a winding or otherprocess without the cost and complexity of making a very thickseparator, the free-standing porous membrane prepared by the methods ofthis invention has a thickness from 6 microns to 25 microns. In anotherembodiment, the microporous inorganic oxide layer of step (b) islaminated to an electrode for an electrochemical cell prior to step (c)and the delamination of step (c) forms the separator laminated to theelectrode. By this method, the mechanical strength requirements forfabrication into an electrochemical cell are provided by the electrode,which is typically coated on an aluminum or copper foil that is 10microns or more in thickness and is mechanically strong. Accordingly,the separator for use in a prelaminate of an electrode and separator mayhave a thickness as low as about 1 micron.

The amount of the pores in the separator may be characterized by thepercent porosity or percent pore volume, which is the cubic centimetersof pores per cubic centimeters of the separator. The porosity may bemeasured by filling the pores with a relatively non-volatile liquidhaving a known density and then calculated by the increase in weight ofthe separator with the liquid present divided by the known density ofthe liquid and then dividing this quotient by the volume of theseparator, as calculated from the area and average thickness of theseparator. In one embodiment of the separators and of the methods ofpreparing separators of this invention, the pore volume of separator isfrom 48 percent to 62 percent. Below a pore volume of 48 percent, theionic conductivity is typically reduced. Above a pore volume of 62percent, the mechanical properties are typically reduced.

The mechanical properties of the separator in the range of 0 percent to2 percent elongation, as expressed by its elastic modulus or Young'smodulus properties and by its tensile strength at 2% elongation, areimportant for efficiency and good yields during the fabrication process,which is typically done by a winding process of combining the electrodesand separator. Once a separator has elongated by more than about 2percent, its width has been lowered significantly and possibly somedistortion has occurred, such that the separator is likely to be nolonger suitable for use in the electrochemical cell due to the enhancedrisk of short circuits. This is true in spite of the extra width usuallyincorporated into the separator compared to the width of the electrodesin order to prevent short circuits on the edges of the electrodes. Inone embodiment of the separators and of the methods of preparingseparators of this invention, the elastic modulus of the separator isfrom 15,000 kg/cm² to 50,000 kg/cm². In one embodiment, the elasticmodulus of the separator is from 30,000 kg/cm² to 70,000 kg/cm². Bycontrast, the elastic modulus of a polyolefin separator is typicallyabout 10,000 to 15,000 kg/cm². In another embodiment, the tensilestrength of the separator at 2 percent elongation is 100 kg/cm² to 500kg/cm². By contrast, the tensile strength of a polyolefin separator istypically about 100 kg/cm² at 2 percent elongation. In one embodiment,the percent elongation of the separator at break is 2 percent to 10percent. In one embodiment, the percent elongation of the separator atbreak is from 5 percent to 20 percent. In one embodiment, the percentelongation of the separator at break is greater than 10 percent. In oneembodiment, the percent elongation of the separator at break is greaterthan 15 percent. An elongation at break above 5%, and preferably above10%, is usually sufficient elongation to indicate good flexibility inthe separator and to protect against brittleness in the separator.

In one embodiment of the separators and of the methods of preparing aseparator of this invention, the average pore diameter of themicroporous inorganic oxide layer is from 2 nm to 70 nm. Typically, theaverage pore diameter of the microporous inorganic oxide layer is from30 to 50 nm. These extremely small pores, that are about 5 to 10 timessmaller than the average pore dimensions of polyolefin separators,present no limitation to high conductivity with lithium saltelectrolytes. Thus, the pore sizes of the separators of this inventionmay provide ion transport and conductivity with lithium-ion batteryelectrolytes that is at least equal to that of polyolefin separators.The divinyl ether of an ethylene glycol and/or organic carbonateadditives in the liquid mixture of the separators and of the methods ofpreparing separators of the present invention are useful in enhancingthis level of ionic conductivity while maintaining or improving theporosity and the mechanical properties of the microporous inorganicoxide separator.

In one embodiment of the separators and of the methods of preparingseparators of the present invention, the organic polymer is present inthe amount of 5 percent to 35 percent of the weight of theorganically-modified inorganic oxide in the microporous inorganic oxidelayer. In a preferred embodiment, the organic polymer is present in theamount of 10 to 15 percent of the weight of the organically-modifiedinorganic oxide in the microporous inorganic oxide layer. These weightratios are typical when the density of the organically-modifiedinorganic oxide is about 3 g/cm³ and should be adjusted to similarvolume percent ratios of the organic significantly higher or lower than3 g/cm³ or if the density of the organic polymer differs significantlyfrom 1.3 g/cm³. At higher polymer levels, the pore volume and ionicconductivity are lowered. At lower polymer levels, the mechanicalproperties are lowered.

In one embodiment of the methods of preparing separators of thisinvention, the inorganic oxide of step (a) is selected from the groupconsisting of pseudo-boehmites, aluminum oxides, silicon oxides, tinoxides, zirconium oxides, and titanium oxides. Preferred inorganicoxides are aluminum boehmite and zirconium oxides. The term“pseudo-boehmite,” as used herein, pertains to hydrated aluminum oxideshaving the chemical formula, Al₂O₃.xH₂O wherein x is in the range of 1.0to 1.5. Terms used herein, which are synonymous with “pseudo-boehmite,”include “aluminum boehmite,” “boehmite,” “AlOOH,” and “hydratedalumina.” The materials referred to herein as “pseudo-boehmite” aredistinct from anhydrous aluminum oxides or aluminas (Al₂O₃ such asalpha-alumina or gamma-alumina) and hydrated aluminum oxides of theformula Al₂O₃.xH₂O wherein x is less than 1.0 or greater than 1.5. Theorganically-modified aluminum oxides of the present invention falloutside of this definition of pseudo-boehmite and instead fall under thedefinition of a hydrated aluminum oxide of the formula Al₂O₃.xH₂Owherein x is less than 1.0, and wherein the hydrated aluminum oxidefurther comprises organic substituents.

In one embodiment of the separators and of the methods of preparingseparators of this invention, the ethylene glycol of the divinyl etheris selected from the group consisting of ethylene glycol, diethyleneglycol, triethylene glycol, and tetraethylene glycol. A preferredethylene glycol is triethylene glycol.

In one embodiment of the separators and of the methods of preparingseparators of the present invention, the organic carbonate is selectedfrom the group consisting of ethylene carbonate, propylene carbonate,butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, vinylenecarbonate, dipropyl carbonate, dibutyl carbonate, and diethyl carbonate.In a preferred embodiment, the organic carbonate is ethylene carbonate.

In one embodiment of the separators and of the methods of preparingseparators of this invention, the organic polymer comprises a polymerselected from the group consisting of polyvinyl alcohols, polyethyleneoxides, polyvinyl pyrrolidones, and cellulosic polymers.

In one embodiment of the methods of preparing a separator of the presentinvention, the drying of step (b) comprises drying at a temperaturegreater than 150° C. In one embodiment, the drying at a temperaturegreater than 150° C. increases the tensile strength of the separator at2 percent elongation and increases the percent elongation of theseparator at break compared to drying for the same period of time at atemperature of 140° C. or less. In one embodiment, the methods furthercomprise a step (d) of drying at a temperature greater than 150° C.

In one embodiment of the methods of this invention, the substrate is atemporary carrier substrate with the surface on which the liquid mixtureis coated on having low adhesion to the dried coating of step (b) suchthat the microporous inorganic oxide layer may be easily delaminatedfrom the substrate without tearing. At the same time, the adhesion mustbe strong enough that the inorganic oxide layer does not prematurelydelaminate from the substrate during the drying process of step (b) orduring mechanical handling to carry out step (c). This balance of enoughadhesion to prevent premature delamination during the microporousinorganic oxide drying process that typically includes some shrinkage ofthe coated layer, together with low adhesion for ease of delamination,may be provided by a variety of substrates, especially those that arevery smooth and non-polar and those that have a release coating on asmooth surface. For example, one suitable substrate is a siliconerelease substrate, such as is used in the coating and delaminationprocess to prepare thin urethane films. The substrate may be a paper,plastic film, metal, or another flexible web substrate.

In one embodiment of the separators and of the methods of preparingseparators of this invention, the liquid mixture to prepare theseparator and the dried separator further comprises a surfactant. Thesurfactant may serve a variety of purposes, such as, for example,helping to disperse the inorganic oxide, aiding in the uniform wettingof the liquid mixture on the substrate, and improving the quality of thexerogel layer by its influence on the liquid-air drying process of themicroporous layer formation. In one embodiment, the surfactant comprisesa fluorosurfactant.

In one embodiment of the separators and of the methods of preparingseparators of the present invention, the microporous inorganic oxidelayer has a thickness of 2 microns to 25 microns.

Provided are separators prepared by the methods of this invention, asdescribed herein. In one embodiment, the separator for anelectrochemical cell separator comprises a microporous inorganic oxidexerogel layer, which xerogel layer comprises an inorganic oxide and anorganic polymer, wherein the inorganic oxide comprises a reactionproduct of a divinyl ether of an ethylene glycol and/or the organiccarbonate with an inorganic oxide. In one embodiment, the polymer of adivinyl ether of an ethylene glycol in the xerogel layer is formed bypolymerization of the divinyl ether of an ethylene glycol in the liquidmix used in coating the microporous inorganic oxide layer of theseparator, and the organic polymer of the inorganic oxide layercomprises this divinyl ether polymer. In one embodiment, the inorganicoxide comprises a reaction product of this divinyl ether polymer and aninorganic oxide. In one embodiment, the separator for an electrochemicalcell comprises a microporous inorganic oxide xerogel layer, whichxerogel layer comprises an inorganic oxide and an organic polymer,wherein the inorganic oxide comprises a reaction product of an organiccarbonate with an inorganic oxide. In one embodiment, the separator foran electrochemical cell comprises a microporous inorganic oxide xerogellayer, which xerogel layer comprises an inorganic oxide, a polyvinylalcohol, and a polyethylene oxide, wherein the inorganic oxide comprisesa reaction product of a divinyl ether of an ethylene glycol or polymerthereof and/or an organic carbonate with an inorganic oxide.

Also provided are electrochemical cells comprising the separators ofthis invention, as described herein. The electrochemical cells comprisean anode and a cathode and a separator interposed between the anode andthe cathode. In one embodiment, the electrochemical cell comprises ananode, a cathode, and a separator interposed between the anode and thecathode, wherein the separator comprises a microporous inorganic oxidelayer, preferably a xerogel layer, which microporous layer comprises aninorganic oxide and an organic polymer, wherein the inorganic oxidecomprises a reaction product of an organic compound, such as a divinylether of an ethylene glycol or polymer thereof and/or an organiccarbonate, with an inorganic oxide. In one embodiment, theelectrochemical cell comprises an anode, a cathode, and a separatorinterposed between the anode and the cathode, wherein the separatorcomprises a microporous inorganic oxide layer, preferably a xerogellayer, which microporous layer comprises an inorganic oxide and anorganic polymer, wherein the inorganic oxide comprises a reactionproduct of an organic carbonate with an inorganic oxide. In oneembodiment, the electrochemical cell comprises an anode, a cathode, anda separator interposed between the anode and the cathode, wherein theseparator comprises a microporous inorganic oxide layer, preferablyxerogel layer, which microporous layer comprises an inorganic oxide, apolyvinyl alcohol, and a polyethylene oxide, wherein the inorganic oxidecomprises a reaction product of a divinyl ether of an ethylene glycol orpolymer thereof and/or an organic carbonate with an inorganic oxide. Inone embodiment of the electrochemical cells of the present invention,the cell comprises lithium as the anode active material. The separatorsof this invention also may function in non-lithium cells, such asalkaline cells. The cells comprising lithium as the anode activematerial include lithium-ion cells and cells with lithium metal as theanode. In one embodiment, the cell is a secondary or rechargeable cell.In one embodiment, the cell is a primary or non-rechargeable cell.

EXAMPLES

Several embodiments of the present invention are described in thefollowing examples, which are offered by way of illustration and not byway of limitation.

Example 1

To further improve the mechanical properties, especially flexibility,without compromising ionic conductivity, several types of additives thatare soluble or dispersible in water and were expected to have good ionicconductivity and compatibility with lithium ion battery chemistry wereevaluated in inorganic oxide xerogel separators where the inorganicoxide was aluminum boehmite. These types of additives were: (1) organiccarbonates, such as ethylene carbonate; (2) divinyl ethers of ethyleneglycol, such as the divinyl ether of triethylene glycol (available asDVE-3 from International Specialty Products, Wayne, N.J.); and (3)polyethylene glycol (PEO), such as PEO with an average molecular weightof 200.

The comparative separator samples with no additives present were made bythe following method. 2.14 grams of glacial acetic acid was added to107.5 grams of distilled water. 20.68 of Dispal 10F4, a tradename for analuminum boehmite powder available from Sasol Corporation, Houston,Tex., was added with stirring to the water/acetic acid mix. 41.06 gramsof an 8.5% solids solution of Celvol 165, a tradename for polyvinylalcohol available from Celanese Corporation, Dallas, Tex., was placed ina separate container containing 0.13 grams of Zonyl FS0-100, a tradenamefor a fluorosurfactant available from E.I. DuPont Corporation,Wilmington, Del., and stirred and heated to above 50° C. The 130.3 gramsof the water/acetic acid/Dispal 10F4 mix was added slowly to the hot andstirred solution of Celvol 165 and Zonyl FS0-100. This mix was coatedwith a #80 wire wound rod onto a 3 mil thick silicone-coated polyesterfilm and dried at 120° C. for 6 minutes in a laboratory convection oven.The resulting microporous coating was delaminated by peeling the layeroff of the silicone release polyester film to give a free-standingmicroporous separator membrane. The separator thickness was measuredwith a Dorsey gauge. The thickness was typically in the range of 16 to20 microns. To make the experimental separator samples with theadditives that enhance ionic conductivity, the additives were added tothe hot and stirring polyvinyl alcohol solution prior to the combinationwith the aluminum boehmite mix.

These three types of additives were evaluated singly and in combinationin the sol gel coating mixes. Surprisingly, these additives made uniformcoating mixes with no gelation that gave very uniform microporouscoatings when coated and dried. The coatings were made using a #80 wirewound rod with approximately 15% solids solutions and coated onto a 3mil thick polyester film on the side that had been previously coatedwith a silicone release layer. The coatings were dried at 120° C. for 6minutes in a laboratory convection oven. The dry thickness of thexerogel separators for measurement of ionic conductivity and mechanicalproperties was in the range of 14 to 22 microns. Also surprisingly, thesolution life of these xerogel or sol gel mixes was greatly extendedfrom 1 to 2 hours to 48 or more hours by the presence of these additivesin the aluminum boehmite sol gel coating mix. This is a very positivefeature for manufacturing xerogel-related coated products because themore stable sol gel mixes mean that special processes typicallyemployed, such as in-line mixing at the coating head of the aluminumboehmite sol solution and the organic polymer solution to make the finalmix and keeping the coating mix temperature above 50° C. to minimizegelation, may not be needed.

Most importantly and surprisingly, these additives significantlyincreased the ionic conductivity of the xerogel separator whilesimultaneously improving the mechanical properties, mainly in the areaof providing more flexibility and reducing brittleness. For example,ethylene carbonate was very effective in providing increased ionicconductivity. In combination with a divinyl ether of an ethylene glycol,such as DVE-3, even a further increase in ionic conductivity wasobserved, and the % elongation of the separator before break increasedfrom about 1% to about 2%. The DVE-3 alone as an additive to thealuminum boehmite sol and polyvinyl alcohol polymer in water alsoincreased the ionic conductivity compared to the control mix with noDVE-3 present. For example, the following ionic conductivity resultswere obtained with separator coating mixes containing 80 parts of Dispal10F4, 15 parts of Celvol 165, 0.5 parts of Zonyl FS0-100, and differentnumbers of relative parts of ethylene carbonate and DVE-3. With mixescontaining 3 parts of DVE-3, increasing the parts of ethylene carbonatefrom 7 to 11 and then to 15 increased the ionic conductivity to 58%,72%, and 105%, respectively, of the ionic conductivity value measuredsimilarly with Celgard 2500 separator, a tradename for a polypropyleneseparator available from PolyPore, Inc., Charlotte, N.C. The ionicconductivity was measured using a HP 4294A impedance analyzer with a1.2M LiBF4 electrolyte solution in 1:1 dimethyl carbonate:ethylenecarbonate. With mixes containing 6.2 parts of DVE-3, increasing theparts of ethylene carbonate from 7 to 11 and then to 15 increased theionic conductivity to 62%, 88%, and 101%, respectively, of the ionicconductivity value measured similarly with Celgard 2500 polypropyleneseparator.

The PEO additive with an average molecular weight of 200 wasparticularly effective in further increasing the % elongation from about2% to about 5% without significantly lowering the ionic conductivity andthe tensile modulus and other mechanical strength properties, includingin cases where the ethylene carbonate and divinyl ether additives werealso present.

Combustion of the various microporous separator samples in a mufflefurnace at about 900° C. for 1 hour after prior vacuum drying at 90° C.for 1 hour, was used to estimate the amount of these additives in thecoating mix that were retained in the microporous separator afterdrying. About 2 to 25% of the ethylene carbonate appeared to be retainedin the dried microporous coating. While not being bound by anyparticular theory, this suggests that some of the positive influence ofthe ethylene carbonate on the microporous separator is from its effecton the porous structure of the separator that is formed during thedrying process. The drying process for sol gels from liquids, theso-called xerogel drying process, is highly influenced by the liquid-airinterface during the drying. A significant amount of ethylene carbonatein the coating mix, such as about 10% by weight of the solids present,that is soluble in water and is a high boiling solvent, would beexpected to influence the nanoporous drying results. In the series ofexperiments that were done, increasing the amount of ethylene carbonatein the sol gel coating mixes gave a progressively increased ionicconductivity. This was also observed with DVE-3, but the increase in theionic conductivity was lower than that achieved with ethylene carbonate.DVE-3 is not soluble in water but appears to be dispersed in water if asurfactant, such as the fluorosurfactant, Zonyl FSO 100, is added to themix. About 10 to 40% of the DVE-3, or a polymer of DVE-3, was estimatedto be retained in the microporous separator after coating, drying, anddelaminating, when analyzed by the muffle furnace combustion technique,which also included heating at 450° C. for 45 minutes to combust theorganic materials without appreciably removing any water of hydrationfrom the inorganic oxide, such as hydrated aluminum oxide. The separatorsamples with these organic additives of Example 1 showed no signs ofmelting at 300° C., 450° C., and even at 900° C., but rather retainedtheir original visual appearance, even as their organic content wascompletely combusted. This supports a xerogel structure for theseseparator samples, even with the organically-modified inorganic oxide.

The PEO was similar to ethylene carbonate in its retention and isconsidered to function in a similar manner to ethylene carbonate in themicroporous coating and drying process. Some PEO and ethylene carbonateare thought to be retained in the microporous separator, perhaps in acomplexed or a reacted state with the aluminum boehmite, and tocontribute directly to the increased ionic conductivity. Similarly,DVE-3 is thought to be retained in the microporous separator, perhaps ina complexed or a reacted state with the boehmite, and with perhaps somein a polymerized state of a divinyl polymer.

As evidence of this complexed or reacted state with the boehmite, Dispal10F4 boehmite powder was measured to be of the formula Al₂O₃.xH₂O wherex is about 1.1. This was done by first drying the Dispal 10F4 at 450° C.for 45 minutes in a muffle furnace to remove any residual or “free”water and any other residuals. This weight loss was about 0.5%. Furtherheating at 900° C. for 45 minutes showed a weight loss of 16.1% whichrelates to about 1.1 moles of H₂O for each mole of Al₂O₃. By contrast,80 parts of Dispal 10F4 powder mixed with 11 parts of ethylene carbonateor with 6.2 parts of DVE-3 in an approximately 20% solids mix in waterand heated at about 80° C. for an hour before drying at 160° C. for 15minutes, both showed about a 5 to 6% weight loss when heated at 450° C.for 45 minutes and a further about 12% weight loss when heated at 900°C. for 45 minutes. This weight loss of about 12% between 450° C. and900° C. heating relates to about 0.8 moles of H₂O for each mole ofAl₂O₃.

Further evidence of the organically-modified inorganic oxide, a sampleof Xerogel Separator #1, as described later in Example 1, after heatingfirst at 160° C. for 6 minutes, showed about a 23% weight loss whenheated at 450° C. for 45 minutes and a subsequent weight loss of about7% when heated at 900° C. for 45 minutes. This relates to about 0.5moles of H₂O for each mole of Al₂O₃.

A main contribution of the PEO is to provide increased elongation beforebreak to the microporous separator. This extra elongation relates tomuch less brittleness in the microporous separator and an increasedability to handle the separator without inducing any tearing or otherdamage.

Besides these three types of additives, another approach to increasingboth the ionic conductivity and mechanical properties was to heat themicroporous separator, either before or after delamination, to atemperature significantly higher than the 120° C. at which the mix istypically dried to form the nanoporous membrane layer. For example, goodresults for increased ionic conductivity and mechanical properties wereobtained by heating either a laminated or a delaminated sample ofaluminum boehmite microporous separator at 160° C. for 6 minutes. Thisis a surprising result since heating a xerogel-type coating would beexpected to make it more brittle, not less brittle, and would beexpected to have no significant effect on the ionic conductivity, ratherthan having a positive effect. On average, this extra heating increasedthe ionic conductivity by about 35% compared to the same microporousseparator with no extra heating. The tensile strength and percentelongation at break also increased, often by 50% or more up to values of10% to over 20%, on the samples with the extra heating compared to thesame microporous separator with no extra heating. No significantshrinkage of the microporous separator occurred during the extra heatingstep so there were no problems with distortion or buckling of themicroporous membrane from these higher heats. The mass loss from vacuumdrying for 1 hour at 90° C. was extremely low at 0.6±0.4%.

The tensile modulus of the hydrated aluminum oxide separators in the keyrange out to 1.5 to 2% elongation continued to be very good relative tothe polyolefin separators, even with these additives that increaseflexibility and % elongation. The porosity or pore volume of sol gelseparators made from a 15% solids blend of 80:13.5:11:5.5:6.2:0.42 byweight of Celvol 165:Dispal 10F4:ethylene carbonate:PEO (averagemolecular weight of about 200):DVE-3:Zonyl FS0-100 (Xerogel Separator#1) in water with 2% glacial acetic acid and coated with a #80 wirewound rod to a dry thickness of 20 microns, was 52%. This porosity valuewas found both when measured by weighing the amount of DVE-3 imbibedinto the pores of the separator and also when measured by mercuryporosimetry and specific surface area analysis of the pores.

At a Dispal 10F4:Celvol 165 ratio of 80:15 in the control xerogelseparators with no additional additives, the % porosity decreased toabout 49%. This approximately 5% decrease in porosity was enough tolower the ionic conductivity by about 25%. At 80:20 and 80:18.78 ratiosof Dispal 10F4:Celvol 165, the porosities were about 43% and 45%,respectively, with corresponding further decreases in ionicconductivity.

The very good elastic modulus or Young's modulus values are shown inChart 1 below for the Xerogel Separator #1 described above. The Young'sModulus of about 30,000 kg/cm² is about 3 times higher than the about10,000 kg/cm² measured for the Young's Modulus of Celgard 2500 plasticseparator, a porous polypropylene separator available from PolyPore,Inc., Charlotte, N.C.

CHART 1 Tensile Parameters of Xerogel Separator #1 Parameter #1 #2 #3mean units Ultimate 194 167 196 186 kg/cm² Strength Young's 37.6 25.031.5 31.2 10³ kg/cm² Modulus Elongation at 3.3% 4.1% 7.5% 5.0% — BreakT.E.A. at 49 585 1297 810 kJ/cm³ Break Thickness 18.7 21.8 19.5 20.0 μmThe “T.E.A” stands for Tensile Energy Absorption and is the area underthe stress-strain curve. The elongation or flexibility of thesemicroporous separators is good with a 5% elongation at break on average.

This application incorporates by reference two U.S. patent applications,entitled “Microporous Separators for Electrochemical Cells,” U.S. patentapplication Ser. No. 11/652,857 published as US 2008/0182174 and“Methods of Preparing Separators for Electrochemical Cells,” U.S. patentapplication Ser. No. 11/652,858 published as US 2007/0189959, both filedon Jan. 12, 2007.

While the invention has been described in detail and with reference tospecific and general embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. An electrochemical cell comprising: (a) an anode;(b) a cathode; (c) a porous separator comprising a free-standinginorganic oxide layer, and wherein said free-standing inorganic oxidelayer comprises: a hydrated aluminum oxide; and an organic polymer thatis covalently bonded to at least a portion of said hydrated aluminumoxide; and (d) an electrolyte selected from a group consisting of liquidorganic electrolytes, gel polymer electrolytes, and solid polymerelectrolytes, wherein the separator is 6 μm to 25 μm thick.
 2. Theelectrochemical cell of claim 1, wherein the separator has an elasticmodulus of 15,000 kg/cm² to 50,000 kg/cm².
 3. The electrochemical cellof claim 1, wherein the separator has a tensile strength between 100kg/cm² to 500 kg/cm² at 2 percent elongation.
 4. The electrochemicalcell of claim 1, wherein an average pore diameter of said inorganicoxide layer of said separator is 30 nm to 50 nm.
 5. The electrochemicalcell of claim 1, wherein said hydrated aluminum oxide comprisesboehmite.
 6. The electrochemical cell of claim 1, wherein said hydratedaluminum oxide is of a formula Al₂O₃.xH₂O, wherein x is in a range of0.8 to less than 1.0.
 7. The electrochemical cell of claim 1, whereinsaid hydrated aluminum oxide is of a formula Al₂O₃.xH₂O, wherein x is ina range of 1.0 to 1.5.
 8. The electrochemical cell of claim 1, wherein aanode comprises lithium as the anode active material.
 9. Theelectrochemical cell of claim 1, wherein the separator does not melt attemperatures below 300° C.
 10. The electrochemical cell of claim 1,wherein the organic polymer is present in an amount of 5 percent to 35percent of the weight of the hydrated aluminum oxide.
 11. Anelectrochemical cell comprising: (a) an anode; (b) a cathode; (c) aporous separator comprising a free-standing inorganic oxide layer, andwherein said free-standing inorganic oxide layer comprises: a hydratedaluminum oxide; and an organic polymer that is covalently bonded to atleast a portion of said hydrated aluminum oxide; and (d) an electrolyteselected from a group consisting of liquid organic electrolytes, gelpolymer electrolytes, and solid polymer electrolytes, wherein an averagepore diameter of said inorganic oxide layer of said separator is 30 nmto 50 nm.
 12. The electrochemical cell of claim 11, wherein theseparator has a tensile strength between 100 kg/cm² to 500 kg/cm² at 2percent elongation.
 13. The electrochemical cell of claim 11, whereinsaid inorganic oxide layer has a % porosity of 48% to 62%.
 14. Theelectrochemical cell of claim 11, wherein said hydrated aluminum oxideis of a formula Al₂O₃.xH₂O, wherein x is in a range of 0.8 to less than1.0.
 15. The electrochemical cell of claim 11, wherein said hydratedaluminum oxide is of a formula Al₂O₃.xH₂O, wherein x is in a range of1.0 to 1.5.
 16. The electrochemical cell of claim 11, wherein theseparator has an elastic modulus of 15,000 kg/cm² to 50,000 kg/cm². 17.The electrochemical cell of claim 11, wherein the anode compriseslithium as a anode active material.
 18. The electrochemical cell ofclaim 11, wherein the separator does not melt at temperatures below 300°C.
 19. The electrochemical cell of claim 11, wherein the organic polymeris present in an amount of 5 percent to 35 percent of the weight of thehydrated aluminum oxide.
 20. The electrochemical cell of claim 11,wherein the separator has a percent elongation at break at least 10%.